1. Field of the Invention
The present invention relates to an image processing apparatus in which color image data is compressed and decompressed.
2. Description of the Related Art
The assignee of the present invention has proposed, in U.S. patent applications Ser. Nos. 738,562 (filed on Jul. 31, 1991) and 872,142 (filed on Apr. 22, 1992), apparatus which keeps the amount of codes per frame at a constant level by maintaining the amount of code data per block at substantially the same level when performing variable-length coding.
The assignee of this invention has also proposed, in U.S. patent application Ser. No. 753,660 (filed on Aug. 30, 1991), a technique in which, in a still video camera, the amount of code data per frame is kept at a constant level in a continuous shooting mode.
In these arrangements, however, because neither the amount of input image data nor the amount of generated code data is considered, the memory for storing the code data is not efficiently used.
A technique in facsimile communication is known which codes data by the MMR coding scheme and converts MMR codes to MR codes in order to communicate with a receiver that does not have an MMR decoder. However, such conventional schemes fail to provide a sufficient compression ratio when processing multivalue data and, in particular, color image data.
The ADCT (Adaptive Discrete Cosine Transform) scheme developed by the JPEG (Joint Photographic Expert Group) is going to be approved as a standard scheme for coding multivalue image data. However, this ADCT scheme is not satisfactory to achieve both high-quality images and high efficiency because the quantization table and Huffman table used in this method are not variable within a page.
Therefore, there is demand for an apparatus which operates according to both the ADCT scheme and an improved version of the ADCT scheme.